The term retains it's German origins as it implies a far more brutal treatment to the more conventional practices of fisting.
Similarly it is used within the Metal culture to indicate aviolent arousal to someone/thing or to express a distaste for someone/something, depending on context. (See example two).
2. "I'd doppelfaust that girl over there!" or alternatively: "look at that hige brutess over yonder, she needs a good doppefausting in her mouth!"